The present invention relates to speed control circuitry for controlling the speed of a fork lift truck and, more particularly, to such circuitry which accurately controls truck speed, while preventing the truck from exceeding a maximum speed dependent upon the height to which the lift forks are raised.
Fork lift trucks of the type controlled by the speed control circuit of the present invention are typically used to transport and handle goods and containers which are relatively large or heavy. Such a lift truck may be powered by an electric d.c. motor which is connected to rechargeable storage batteries carried on the truck. A pair of lift forks are mounted on an extendable fork mast for engaging the goods or containers being handled. The forks are raised as the goods or contaiers are carried by the truck. Additionally, the forks may be raised quite high, depending upon the construction of the truck, in order to position the goods or containers on a storage rack or to remove the goods or containers from such a rack.
It will be appreciated that transporting a load at a high rate of speed with the forks raised substantially above ground level may be somewhat dangerous. The overall center of gravity of the truck and the load is raised, thus reducing the stability of the truck.
A number of speed control arrangements have been suggested in the past for limiting the maximum speed attainable by a lift truck with the forks raised. Some arrangements have included a switch to sense when the forks are raised above a selected height, with the switch being connected in the electric motor power supply circuit, to prevent the application of full voltage to the driving motor when the forks are elevated.
Such arrangements have the disadvantage that they also limit the maximum power that can be applied to the motor at start up when the truck is completely stopped and the forks are raised. One approach to solution of this problem is shown in U.S. Pat. No. 2,790,513, issued Apr. 30, 1957, to Draxler. The Draxler patent discloses an electromechanical control arrangement in which a relay in the power supply circuit for the motor detects when the motor is approaching a stall condition and switches out the power limiting portion of the circuit.
The Draxler system has the disadvantage that only a single height level for the forks is detected to set a maximum permissible power level for the motor. As a result, the limiting portion of the circuit is either on or off. Additionally, the circuit does not provide true speed control, but rather limits the power supplied to the drive motor, regardless of the speed of the truck.
Accordingly, it is seen that there is a need for a speed control circuit for a lift truck which limits the speed of the truck in dependence upon the height to which the forks are raised and in which the speed of the truck is accurately limited and for a speed control circuit in which operation of the circuit is monitored and discontinued if a malfunction occurs.